U.S. patent number 6,961,364 [Application Number 09/551,078] was granted by the patent office on 2005-11-01 for base station identification in orthogonal frequency division multiplexing based spread spectrum multiple access systems.
This patent grant is currently assigned to Flarion Technologies, Inc.. Invention is credited to Rajiv Laroia, Junyi Li, Sundeep Rangan, Pramod Viswanath.
United States Patent |
6,961,364 |
Laroia , et al. |
November 1, 2005 |
Base station identification in orthogonal frequency division
multiplexing based spread spectrum multiple access systems
Abstract
A base station having the strongest downlink signal is
identified by utilizing a unique slope of a pilot tone hopping
sequence being transmitted by a base station. Specifically, base
station identification is realized by determining the slope of the
strongest received pilot signal, i.e., the received pilot signal
having the maximum energy. In an embodiment of the invention, the
pilot tone hopping sequence is based on a Latin Squares sequence.
With a Latin Squares based pilot tone hopping sequence, all a
mobile user unit needs is to locate the frequency of the pilot
tones at one time because the pilot tone locations at subsequent
times can be determined from the slope of the Latin Squares pilot
tone hopping sequence. The slope and initial frequency shift of the
pilot tone hopping sequence with the strongest received power is
determined by employing a unique maximum energy detector.
Inventors: |
Laroia; Rajiv (Basking Ridge,
NJ), Li; Junyi (Matawan, NJ), Rangan; Sundeep
(Hoboken, NJ), Viswanath; Pramod (Berkeley, CA) |
Assignee: |
Flarion Technologies, Inc.
(Bedminster, NJ)
|
Family
ID: |
24199749 |
Appl.
No.: |
09/551,078 |
Filed: |
April 18, 2000 |
Current U.S.
Class: |
375/132;
375/E1.037; 375/E1.035 |
Current CPC
Class: |
H04L
27/2613 (20130101); H04B 1/7143 (20130101); H04L
27/2657 (20130101); H04B 1/7156 (20130101); H04L
5/0019 (20130101); H04L 27/2675 (20130101); H04L
5/0048 (20130101); H04B 2001/71563 (20130101); H04L
25/0226 (20130101) |
Current International
Class: |
H04L
27/26 (20060101); H04L 5/02 (20060101); H04B
1/69 (20060101); H04B 1/713 (20060101); H04B
001/713 () |
Field of
Search: |
;375/130,132-133,136-138,260 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
2298360 |
|
Sep 2000 |
|
CA |
|
WO 97/26742 |
|
Jul 1997 |
|
WO |
|
Other References
F Tufvesson et al., "Pilot Assisted Estimation for OFDM in Mobile
Cellular Systems", IEEE, 0-7803-3659-3/97, pp. 1639-1643. .
R. Negi et al., "Pilot Tone Selection for Channel Estimation in a
Mobile OFDM System", IEEE Transactions on Consumer Electronics, pp.
1122-1128, 1998. .
EPC Search Report for European Application No. 01303316, Apr. 9,
2001. .
Fernandez-Getino Garcia J et al.: "Efficient Pilot Patterns for
Channel Estimation in OFDM Systems Over HF Channels", VTC
1999-Fall, IEEE VTS 50.sup.th, Vehicular Technology Conference,
Gateway to the 21.sup.st, Century Communications Village,
Amsterdam, Sep. 19-22, 1999, IEEE Vehicular Technology Conference,
New York, NY, U.S.A., vol. 4, Conf 50, Sep. 19, 1999, pp.
2193-2197. .
Wang C C et al: "Dynamic Channel Resource Allocation in Frequency
Hopped Wireless Communication Systems", Information Theory, 1994.
Proceedings, 1994 IEEE International Symposium on Trondheim, Norway
27, Jun. 1, Jul. 1994, New York, NY, U.S.A., IEEE, Jun. 27, 1994,
p. 229. .
Han D S et al: "On the Synchronization of MC-CDMA System for Indoor
Wireless Communications", VTC 1999-Fall, IEEE VTS 50.sup.th,
Vehicular Technology Conference; Gateway to the 21.sup.st Century
Communications Village, Amsterdam, Sep. 19-22, 1999, IEEE Vehicular
Technology Conference, New York, NY, U.S.A, vol. 2, Conf. 50, Sep.
1999, pp. 693-697. .
Fazel K et al: "A Flexible and High Performance Cellular Mobile
Communications System Based on Orthogonal Multi-Carrier SSMA",
Wireless Personal Communications, Kluwer Academic Publishers, NL,
vol. 2, No. 1/2, 1995, pp. 121-144..
|
Primary Examiner: Burd; Kevin
Attorney, Agent or Firm: Straub & Pokotylo Straub;
Michael P.
Claims
What is claimed is:
1. Apparatus for use in a mobile user unit in an orthogonal
frequency division multiplexing (OFDM) based spread spectrum
multiple access wireless system including at least two adjacent
base stations, each one of the adjacent base stations transmitting
pilot tones according to one of a plurality of different pilot tone
hopping sequences over at least a portion of a pilot sequence
transmission time period, said portion including multiple symbol
time periods, at least one of the different pilot tone hopping
sequences including at least two pilot tones per symbol time period
which are separated from one another by at least one tone during
said portion of said pilot sequence transmission time period, in
each of the different pilot tone hopping sequences the number of
pilot tones used in each successive symbol time periods in said
portion of said pilot sequence transmission period being the same
but the tones used in a symbol time period by any one of the
different pilot tone hopping sequences changing in frequency from
one symbol time period to the next symbol time period by a
frequency shift corresponding to a fixed number of tones, adjacent
base stations using different frequency shifts to generate pilot
tone hopping sequences with different pilot tone slopes which can
be determined from the frequency shift of the pilot tones used in
consecutive symbol time periods, the apparatus comprising: a
receiver for receiving one or more of said plurality of different
pilot tone hopping sequences having different pilot tone slopes;
and a detector, responsive to said one or more received pilot tone
hopping sequences, said detector including an energy accumulator
for generating an accumulated energy measurement for each
individual one of the plurality of pilot tone hoping sequences
having different slopes over a period including multiple symbol
time periods, said detector detecting a received pilot tone hopping
sequence having the maximum accumulated energy over said period
including multiple symbol time periods.
2. The invention as defined in claim 1 wherein each of said one or
more received pilot tone hopping sequences is a Latin Squares based
pilot tone hopping sequence.
3. The invention as defined in claim 1 wherein said receiver yields
a baseband version of a received signal and further includes a unit
for generating a fast Fourier transform version of said baseband
signal, and wherein said detector is supplied with said fast
Fourier transform version of said baseband signal to detect, based
on accumulated energy measurements, the received pilot tone
sequence having the maximum accumulated energy.
4. The invention as defined in claim 3 wherein said receiver
further includes a quantizer for quantizing the results of said
fast Fourier transform.
5. The invention as defined in claim 3 wherein said detector is a
maximum energy detector.
6. The invention as defined in claim 5, wherein different initial
frequency shifts are possible for different pilot tone hopping
sequences having the same slope; and wherein said maximum energy
detector determines a slope and an initial frequency shift for
pilot tones in the detected pilot tone hopping sequence having the
maximum accumulated energy.
7. The method of claim 1, wherein frequency spacing between pilot
tones which occur in a symbol time period in each of said plurality
of tone hopping sequences is fixed and is the same for all of said
plurality of pilot tone hopping sequences.
8. Apparatus for use in a mobile user unit in an orthogonal
frequency division multiplexing (OFDM) based spread spectrum
multiple access wireless system comprising: a receiver for
receiving one or more pilot tone hopping sequences each including
pilot tones, said pilot tones each being generated at a prescribed
frequency and time instants in a prescribed time-frequency grid;
and a maximum energy detector, responsive to said one or more
received pilot tone hopping sequences, for detecting the received
pilot tone hopping sequence having the strongest power, said
maximum energy detector including a slope-shift accumulator for
accumulating energy along each possible slope and initial frequency
shift of said one or more received pilot tone hopping sequences and
generating an accumulated energy signal, a frequency shift
accumulator supplied with said accumulated energy signal for
accumulating energy along pilot frequency shifts of said one or
more received pilot tone hopping sequences, and a maximum detector
supplied with an output from said frequency shift accumulator for
estimating a slope and initial frequency shift of the strongest
received pilot tone hopping sequence as a slope and initial
frequency shift corresponding to the strongest accumulated
energy.
9. The invention as defined in claim 8 wherein said accumulated
energy is represented by the signal J.sub.0 (s, b.sub.0), where
##EQU10##
and s is the slope of the pilot signal, b.sub.0 is an initial
frequency shift of the pilot signal, Y(t,n) is the fast Fourier
transform data, t=0, . . . , N.sub.sy -1, n=st+b.sub.0 (mod N), and
n=0, . . . N-1.
10. The invention as defined in claim 8 wherein said frequency
shift accumulator accumulates energy along pilot frequency shifts
of said one or more received pilot tone hopping sequences in
accordance with ##EQU11##
where s is the slope of the pilot signal, b.sub.0 is an initial
frequency shift of the pilot signal and n.sub.j are frequency
offsets.
11. The invention as defined in claim 8 wherein said maximum
detector estimates said slope and initial frequency shift of the
strongest received pilot tone hopping sequence in accordance with
##EQU12##
where s is the estimate of the slope, b.sub.0 is the estimate of
the initial frequency shift, and where the maximum is taken over
s.epsilon.S and b.sub.0 =0, . . . ,N-1.
12. Apparatus for use in a mobile user unit in an orthogonal
frequency division multiplexing (OFDM) based spread spectrum
multiple access wireless system comprising: a receiver for
receiving one or more pilot tone hopping sequences each including
pilot tones, said pilot tones each being generated at a prescribed
frequency and time instants in a prescribed time-frequency grid;
and a maximum energy detector, responsive to said one or more
received pilot tone hopping sequences, for detecting the received
pilot tone hopping sequence having the strongest power, said
maximum energy detector including a frequency shift detector for
estimating at a given time frequency shift of the received pilot
tone hopping sequence having strongest energy and an estimated
maximum energy value, and a slope and frequency shift solver,
responsive to said estimated frequency shift and said estimated
maximum energy value, for generating estimates of an estimated
slope and an estimated initial frequency shift of the strongest
received pilot signal.
13. The invention as defined in claim 12 wherein said estimated
frequency shift at time t is obtained in accordance with
n(t)=st+b.sub.0 (mod N), where s is the pilot signal slope, t is a
symbol time and n(t) is a frequency shift estimate.
14. The invention as defined in claim 13 wherein said estimated
maximum energy value is obtained in accordance with ##EQU13##
where E(t) is the maximum energy value, Y(t,n) is the fast Fourier
transform data, j=1, . . . , N.sub.p and n.sub.j are frequency
offsets.
15. The invention as defined in claim 14 wherein said slope is
estimated in accordance with ##EQU14##
where both n(t) and n(t-1) satisfy n(t)=st+b.sub.0 (mod N).
16. The invention as defined in claim 14 wherein said frequency
shift is estimated in accordance with ##EQU15##
17. The invention as defined in claim 12 wherein said maximum
energy detector detects said slope in accordance with determining
the time, t.sub.0.epsilon.T, and slope, s.sub.0.epsilon.S, such
that the set of times t on the line n(t)=n(t.sub.0)+s.sub.0
(t-t.sub.0), has the largest total pilot signal energy.
18. A method for use in a mobile user unit in an orthogonal
frequency division multiplexing (OFDM) based spread spectrum
multiple access wireless system including at least two adjacent
base stations, each one of the adjacent base stations transmitting
pilot tones according to one of a plurality of different pilot tone
hopping sequences, in each of the different pilot tone hopping
sequences over at least a portion of a pilot sequence transmission
time period, said portion including multiple symbol time periods,
the number of pilot tones used in each successive symbol time
period in said portion of said pilot sequence transmission time
period being the same but the tones used in a symbol time period by
any one of the different pilot tone hopping sequences changing in
frequency from one symbol time period to the next symbol time
period by a frequency shift corresponding to a fixed number of
tones, adjacent base stations using different frequency shifts to
generate pilot tone hoping sequences with different pilot tone
slopes which can be determined from the frequency shift of the
pilot tones used in consecutive symbol time periods, the method
comprising the steps of: receiving one or more of said plurality of
different pilot tone hopping sequences having different pilot tone
hopping slopes; and in response to said one or more received pilot
tone hopping sequences: generating an accumulated energy
measurement for each individual one of the plurality of pilot tone
hoping sequences having different pilot tone hopping slopes over a
period including multiple symbol time periods; and detecting a
received pilot tone hopping sequence having the maximum accumulated
energy over said period including multiple symbol time periods.
19. The method as defined in claim 18 wherein each of said one or
more received pilot tone hopping sequences is a Latin Squares based
pilot tone hopping sequence.
20. The method as defined in claim 18 wherein said step of
receiving yields a baseband version of a received signal and
further including a step of generating a fast Fourier transform
version of said baseband signal, and wherein said step of detecting
is responsive to said fast Fourier transform version of said
baseband signal for detecting the received pilot tone sequence
having the maximum accumulated energy.
21. The method as defined in claim 20 wherein said step of
receiving further includes a step of quantizing the results of said
fast Fourier transform.
22. The method as defined in claim 20 wherein said step of
detecting detects a maximum energy.
23. The method as defined in claim 22 wherein said step of
detecting said maximum energy includes a step of determining a
slope and initial frequency shift of pilot tones in a detected
pilot tone hopping sequence having the maximum accumulated
energy.
24. A method for use in a mobile user unit in an orthogonal
frequency division multiplexing (OFDM) based spread spectrum
multiple access wireless system comprising the steps of: receiving
one or more pilot tone hopping sequences each including pilot
tones, said pilot tones each being generated at a prescribed
frequency and time instants in a prescribed time-frequency grid;
and in response to said one or more received pilot tone hopping
sequences, detecting the received pilot tone hopping sequence
having the maximum energy, said step of detecting said maximum
energy including the steps of accumulating energy along each
possible slope and initial frequency shift of said one or more
received pilot tone hopping sequences and generating an accumulated
energy signal, in response to said accumulated energy signal,
accumulating energy along pilot frequency shifts of said one or
more received pilot tone hopping sequences, and in response to an
output from said step of frequency shift accumulating, estimating a
slope and initial frequency shift of the strongest received pilot
tone hopping sequence as a slope and initial frequency shift
corresponding to the strongest accumulated energy.
25. The method as defined in claim 24 wherein said accumulated
energy is represented by the signal J.sub.0 (s,b.sub.0), where
##EQU16##
and s is the slope of the pilot signal, b.sub.0 is an initial
frequency shift of the pilot signal, Y(t,n) is the fast Fourier
transform data, t=0, . . . , N.sub.sy -1, n=st+b.sub.0 (mod N), and
n=0, . . . N-1.
26. The method as defined in claim 24 wherein said step of
frequency shift accumulating includes a step of accumulating energy
along pilot frequency shifts of said one or more received pilot
tone hopping sequences in accordance with ##EQU17##
where s is the slope of the pilot signal, b.sub.0 is an initial
frequency shift of the pilot signal and n.sub.j are frequency
offsets.
27. The method as defined in claim 24 wherein said step of maximum
energy detecting includes a step of estimating said slope and
initial frequency shift of the strongest received pilot tone
hopping sequence in accordance with ##EQU18##
where s is the estimate of the slope, b.sub.0 is the estimate of
the initial frequency shift, and where the maximum is taken over
s.epsilon.S and b.sub.0 =0, . . . ,N-1.
28. A method for use in a mobile user unit in an orthogonal
frequency division multiplexing (OFDM) based spread spectrum
multiple access wireless system comprising the steps of: receiving
one or more pilot tone hopping sequences each including pilot
tones, said pilot tones each being generated at a prescribed
frequency and time instants in a prescribed time-frequency grid;
and in response to said one or more received pilot tone hopping
sequences, detecting the received pilot tone hopping sequence
having maximum energy, said step of detecting the received pilot
tone hopping sequence having maximum energy including a step of
estimating, at a given time, a frequency shift of the received
pilot tone hopping sequence having maximum energy and estimating a
maximum energy value, and in response to said estimated frequency
shift and said estimated maximum energy value, generating estimates
of an estimated slope and an estimated initial frequency shift of
the strongest received pilot signal.
29. The method as defined in claim 28 wherein said estimated
frequency shift at time t is obtained in accordance with
n(t)=st+b.sub.0 (mod N), where s is the pilot signal slope, t is a
symbol time and n(t) is a frequency shift estimate.
30. The method as defined in claim 29 wherein said estimated
maximum energy value is obtained in accordance with ##EQU19##
where E(t) is the maximum energy value, Y(t,n) is the fast Fourier
transform data, j=1, . . . , N.sub.p and n.sub.j are frequency
offsets.
31. The method as defined in claim 30 wherein said slope is
estimated in accordance with ##EQU20##
where both n(t) and n(t-1) satisfy n(t)=st+b.sub.0 (mod N).
32. The method as defined in claim 30 wherein said frequency shift
is estimated in accordance with ##EQU21##
33. The method as defined in claim 28 wherein said step of maximum
energy detecting includes a step of finding the time,
t.sub.0.epsilon.T, and slope, s.sub.0.epsilon.S, such that the set
of times t on the line n(t)=n(t.sub.0)+s.sub.0 (t-t.sub.0), has the
largest total pilot signal energy.
34. Apparatus for use in a mobile user unit in an orthogonal
frequency division multiplexing (OFDM) based spread spectrum
multiple access wireless system including at least two adjacent
base stations, each one of the adjacent base stations transmitting
pilot tones according to one of a plurality of different pilot tone
hopping sequences over at least a portion of a pilot sequence
transmission time period, said portion including multiple symbol
time periods, at least one of the different pilot tone hopping
sequences including at least two pilot tones per symbol time period
which are separated from one another by at least one tone during
said portion of said pilot sequence transmission time period, in
each of the different pilot tone hopping sequences the number of
pilot tones used in each successive symbol time period in said
portion of said pilot sequence transmission time period being the
same but the tones used in a symbol time period by any one of the
different pilot tone hopping sequences changing in frequency from
one symbol time period to the next symbol time period by a
frequency shift corresponding to a fixed number of tones, adjacent
base stations using different frequency shifts to generate pilot
tone hopping sequences with different pilot tone slopes which can
be determined from the frequency shift of the pilot tones used in
consecutive symbol time periods, the apparatus comprising: means
for receiving one or more of said different pilot tone hopping
sequences each including pilot tones; and means, responsive to said
one or more received pilot tone hopping sequences, for generating
an accumulated energy measurement for each individual one of the
plurality of different pilot tone hoping sequences having different
pilot tone slopes; and detector means for detecting a received
pilot tone hopping sequence having the maximum accumulated energy
over a period including multiple symbol time periods.
35. The invention as defined in claim 34 wherein each of said one
or more received pilot tone hopping sequences is a Latin Squares
based pilot tone hopping sequence.
36. The invention as defined in claim 34 wherein said means for
receiving yields a baseband version of a received signal and
further including means for generating a fast Fourier transform
version of said baseband signal, and wherein said means for
detecting is responsive to said fast Fourier transform version of
said baseband signal for determining a received pilot tone sequence
having the maximum energy.
37. The invention as defined in claim 36 wherein said means for
generating said fast Fourier transform includes means for
quantizing the results of said fast Fourier transform.
38. The invention as defined in claim 36 wherein means for
detecting detects a maximum energy.
39. The invention as defined in claim 38 wherein said means for
detecting said maximum energy includes means for determining a
slope and an initial frequency shift of pilot tones in a detected
pilot tone hopping sequence having the maximum energy.
40. Apparatus for use in a mobile user unit in an orthogonal
frequency division multiplexing (OFDM) based spread spectrum
multiple access wireless system comprising the steps of: means for
receiving one or more pilot tone hopping sequences each including
pilot tones, said pilot tones each being generated at a prescribed
frequency and time instants in a prescribed time-frequency grid;
and means, responsive to said one or more received pilot tone
hopping sequences, for detecting the received pilot tone hopping
sequence having maximum energy, said means for detecting said
maximum energy including means for accumulating energy along each
possible slope and initial frequency shift of said one or more
received pilot tone hopping sequences, means for generating an
accumulated energy signal, means, responsive to said accumulated
energy signal, for accumulating energy along pilot frequency shifts
of said one or more received pilot tone hopping sequences, and
means, responsive to an output from said means for frequency shift
accumulating, for estimating a slope and an initial frequency shift
of the strongest received pilot tone hopping sequence as the slope
and the initial frequency shift corresponding to the strongest
accumulated energy.
41. The invention as defined in claim 40 wherein said accumulated
energy is represented by the signal J.sub.0 (s.sub.1 b.sub.0),
where ##EQU22##
and s is the slope of the pilot signal, b.sub.0 is an initial
frequency shift of the pilot signal, Y(t,n) is the fast Fourier
transform data, t=0, . . . N.sub.sy -1, n=st+b.sub.0 (mod N), and
n=0, . . . N-1.
42. The invention as defined in claim 40 wherein said means for
frequency shift accumulating includes means for accumulating energy
along pilot frequency shifts of said one or more received pilot
tone hopping sequences in accordance with ##EQU23##
where s is the slope of the pilot signal, b.sub.0 is an initial
frequency shift of the pilot signal and n.sub.j are frequency
offsets.
43. The invention as defined in claim 40 wherein said means for
maximum energy detecting includes means for estimating said slope
and initial frequency shift of the strongest received pilot tone
hopping sequence in accordance with ##EQU24##
where s is the estimate of the slope, b.sub.0 is the estimate of
the initial frequency shift, and where the maximum is taken over
s.epsilon.S and b.sub.0 =0, . . . ,N-1.
44. Apparatus for use in a mobile user unit in an orthogonal
frequency division multiplexing (OFDM) based spread spectrum
multiple access wireless system comprising the steps of: means for
receiving one or more pilot tone hopping sequences each including
pilot tones, said pilot tones each being generated at a prescribed
frequency and time instants in a prescribed time-frequency grid;
and means, responsive to said one or more received pilot tone
hopping sequences, for detecting the received pilot tone hopping
sequence having maximum energy, said means for detecting said
maximum energy including means for estimating at a given time a
frequency shift of the received pilot tone hopping sequence having
maximum energy and for estimating a maximum energy value, and
means, responsive to said estimated frequency shift and said
estimated maximum energy value, for generating estimates of an
estimated slope and an estimated initial frequency shift of the
strongest received pilot signal.
45. The invention as defined in claim 44 wherein said estimated
frequency shift at time t is obtained in accordance with
n(t)=st+b.sub.0 (mod N), where s is the pilot signal slope, t is a
symbol time and n(t) is a frequency shift estimate.
46. The invention as defined in claim 45 wherein said estimated
maximum energy value is obtained in accordance with ##EQU25##
where E(t) is the maximum energy value, Y(t,n) is the fast Fourier
transform data, j=1, . . . , N.sub.p and n.sub.j are frequency
offsets.
47. The invention as defined in claim 46 wherein said slope is
estimated in accordance with ##EQU26##
where both n(t) and n(t-1) satisfy
n(t)=st+b.sub.0 (mod N).
48. The invention as defined in claim 46 wherein said frequency
shift is estimated in accordance with ##EQU27##
49. The invention as defined in claim 44 wherein said means for
detecting maximum energy includes means for finding the time,
t.sub.0.epsilon.T, and slope, s.sub.0.epsilon.S, such that the set
of times t on the line n(t)=n(t.sub.0)+s.sub.0 (t-t.sub.0), has the
largest total pilot signal energy.
50. An orthogonal frequency division multiplexing (OFDM) based
spread spectrum multiple access wireless system comprising: at
least two adjacent base stations, each one of the adjacent base
stations transmitting pilot tones according to one of a plurality
of different pilot tone hopping sequences over at least a portion
of a pilot sequence transmission time period, said portion
including multiple symbol time periods, at least one of the
different pilot tone hopping sequences including at least two pilot
tones per symbol time period which are separated from one another
by at least one tone during said portion of said pilot sequence
transmission time period, in each of the different pilot tone
hopping sequences the number of pilot tones used in each successive
symbol time period in said portion of said pilot sequence
transmission period being the same but the tones used in a symbol
time period by any one of the different pilot tone hopping
sequences changing in frequency from one symbol time period to the
next symbol time period by a frequency shift corresponding to a
fixed number of tones, adjacent base stations using different
frequency shifts to generate pilot tone hopping sequences with
different pilot tone slopes which can be determined from the
frequency shift of the pilot tones used in consecutive symbol time
periods; and a mobile communications device including: i) a
receiver for receiving one or more of said plurality of different
pilot tone hopping sequences; and ii) means for determining the
pilot tone slope of a received pilot tone hopping sequence.
51. An orthogonal frequency division multiplexing (OFDM) based
spread spectrum multiple access wireless communications method,
comprising: at least two adjacent bases stations which transmit
pilot tones according to different ones of a plurality of different
pilot tone hopping sequences over at least a portion of a pilot
sequence transmission time period, said portion including multiple
symbol time periods, at least one of the different pilot tone
hopping sequences including at least two pilot tones per symbol
time period which are separated from one another by at least one
tone during said portion of said pilot sequence transmission time
period, in each of the different pilot tone hopping sequences the
number of pilot tones used in each successive symbol time period in
said portion of said pilot sequence transmission period being the
same but the tones used in a symbol time period by any one of the
different pilot tone hopping sequences changing in frequency from
one symbol time period to the next symbol time period by a
frequency shift corresponding to a fixed number of tones, each of
the adjacent base stations using different frequency shifts to
generate the transmitted pilot tone hopping sequences resulting in
different pilot tone slopes which can be determined from the
frequency shift of the pilot tones transmitted in consecutive
symbol time periods.
52. The method of claim 51, wherein frequency spacing between pilot
tones which occur in a symbol time period in each of said plurality
of tone hopping sequences is fixed and is the same for all of said
plurality of pilot tone hopping sequences.
Description
RELATED APPLICATION
This application is related U.S. patent application Ser. No.
09/551,791 which was filed on Apr. 18, 2000.
TECHNICAL FIELD
This invention relates to wireless communications systems and, more
particularly, to orthogonal frequency division multiplexing (OFDM)
based spread spectrum multiple access (SSMA) systems.
BACKGROUND OF THE INVENTION
It is important that wireless communications systems be such as to
maximize the number of users that can be adequately served and to
maximize data transmission rates, if data services are provided.
Wireless communications systems are typically shared media systems,
i.e., there is a fixed available bandwidth that is shared by all
users of the wireless system. Such wireless communications systems
are often implemented as so-called "cellular" communications
systems, in which the territory being covered is divided into
separate cells, and each cell is served by a base station.
In such systems, it is important that mobile user units are rapidly
able to identify and synchronize to the downlink of a base station
transmitting the strongest signal. Prior arrangements have
transmitted training symbols periodically for mobile user units to
detect and synchronize to the associated base station downlink. In
such arrangements, there is a large probability that delays occur
in identifying the base station transmitting the strongest signal
because the training symbols are typically transmitted at the
beginning of a frame. It is also likely that the training symbols
transmitted from different base stations would interfere with each
other. Indeed, it is known that once the training symbols interfere
with each other they will continue to interfere. Thus, if the
training symbols are corrupted, then the data is also corrupted,
thereby causing loss in efficiency.
SUMMARY OF THE INVENTION
Problems and/or limitations related to prior mobile user units that
have attempted to identify a base station having the strongest
downlink signal are addressed by utilizing a pilot tone hopping
sequence being transmitted by a base station. Specifically, base
station identification is realized by determining the slope of the
strongest received pilot signal, i.e., the received pilot signal
having the maximum energy.
In an embodiment of the invention, the pilot tone hopping sequence
is based on a Latin Squares sequence. With a Latin Squares based
pilot tone hopping sequence, all a mobile user unit needs is to
locate the frequency of the pilot tones at one time because the
pilot tone locations at subsequent times can be determined from the
unique slope of the Latin Squares pilot tone hopping sequence. The
slope and initial frequency shift of the pilot tone hopping
sequence with the strongest received power is determined by
employing a unique maximum energy detector. This unique slope of
the pilot tone hopping sequence is then advantageously employed to
identify the base station having the strongest downlink signal.
In one embodiment, the slope and initial frequency shift of the
pilot signal having the strongest received power is determined by
finding the slope and initial frequency shift of a predicted set of
pilot tone locations having the maximum received energy.
In another embodiment, the frequency offset of the pilot signal
with the strongest, i.e., maximum, received power is estimated at
each of times "t". These frequency offsets are employed in
accordance with a prescribed relationship to determine the unknown
slope and the initial frequency shift of the pilot signal.
A technical advantage to using the pilot tone hopping sequence to
identify the base station having the strongest downlink signal is
that the inherent latency resulting from using a sequence of
training symbols is not present.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 illustrates a frequency domain representation in which a
prescribed plurality of tones is generated in a prescribed
bandwidth;
FIG. 2 illustrates a time domain representation of a tone
.function..sub.i ;
FIG. 3 is a graphical representation of a time-frequency grid
including a pilot tone hopping sequence;
FIG. 4 is a graphical representation of a Latin Squares hopping
sequence;
FIG. 5 shows, in simplified block form, an OFDM-SSMA cellular
system with Latin Squares pilots;
FIG. 6 shows, in simplified block diagram form, details of a mobile
user unit in which an embodiment of the invention may
advantageously be employed;
FIG. 7 shows, in simplified block diagram for, details of an
embodiment of a maximum energy detector that may be employed in the
mobile user unit of FIG. 6;
FIG. 8 shows, in simplified block diagram form, details of another
embodiment of a maximum energy detector that may be employed in the
mobile user unit of FIG. 6; and
FIG. 9 is a flow chart illustrating steps in a process that may be
employed in the slope-shift solver of FIG. 8.
DETAILED DESCRIPTION
FIG. 1 illustrates a frequency domain representation in which a
prescribed plurality of tones is generated in a prescribed
bandwidth. In this example, bandwidth W is employed to generate a
total of N tones, i.e., i=1, . . . , N. The tones are spaced at
.DELTA..function.=1/T.sub.S apart, where T.sub.s is the duration of
an OFDM symbol. Note that the tones employed in this embodiment of
the invention are generated differently than those generated for a
narrow band system. Specifically, in a narrow band system the
energy from each tone is strictly confined to a narrow bandwidth
centered around the tone frequency, whereas in an Orthogonal
Frequency Division Multiplexing (OFDM) system that is a wide band
system the energy at a particular tone is allowed to leak into the
entire bandwidth W, but it is so arranged that the tones do not
interfere with one another.
FIG. 2 illustrates a time domain representation of tone
.function..sub.i within symbol interval T.sub.s. Again, note that
within each symbol interval T.sub.s, data may be transmitted on
each of the tones substantially simultaneously.
FIG. 3 is a graphical representation of an example OFDM channel
from a base station to a number of mobile user units, i.e.,
receivers. The OFDM channel is represented as a time-frequency
grid, i.e., plane. Each column of the grid represents the time
interval for one OFDM symbol interval, and each OFDM symbol is
comprised of a number of tones. In this example, there are N=5
tones in each symbol interval. The tones are numbered along the
frequency axis and the symbol intervals, i.e., periods, are
numbered along the time axis. If the spacing between tones in FIG.
3 is .DELTA..function., then:
tone 0 corresponds to .function.;
tone 1 corresponds to .function.+.DELTA..function.;
tone 2 corresponds to .function.+2.DELTA..function.;
tone 3 corresponds to .function.+3.DELTA..function.;
tone 4 corresponds to .function.+4.DELTA..function..
Similarly, if the duration of a symbol interval is T.sub.s
then:
time 0 corresponds to t.sub.0 ;
time 1 corresponds to t.sub.0 +T.sub.s ;
time 2 corresponds to t.sub.0 +2 T.sub.s ;
time 3 corresponds to t.sub.0 +3 T.sub.s ;
time 4 corresponds to t.sub.0 +4 T.sub.s ;
time 5 corresponds to t.sub.0 +5 T.sub.s ;
time 6 corresponds to t.sub.0 +6 T.sub.s.
In general, a pilot signal includes known waveforms that are
transmitted from a base station so that mobile user units, i.e.,
receivers, can identify the base station and estimate various
channel parameters. In an Orthogonal Frequency Division
Multiplexing based Spread Spectrum Multiple Access (OFDM-SSMA)
system, in accordance with an aspect of the invention, the pilot
signal is comprised of known symbols transmitted on prescribed
tones during prescribed symbol intervals. In a given symbol
interval, the tones used for the pilot signal are called the "pilot
tones", and the assignment of pilot tones as a function of time is
called the "pilot hopping sequence". Again, it is noted that the
inherent delays resulting when using the training sequence of
symbols is not experienced when using the pilot tone hopping
sequence to identify the base station having the strongest downlink
signal.
Since the OFDM-SSMA physical layer is based on the pilot signals,
symbols on the pilot tones are transmitted at higher power than
symbols on non-pilot tones. Pilot tones are also boosted in power
so that they may be received throughout the cell. Therefore, for
the purpose of identification, pilot signals can be distinguished
by the fact that the energy received on the pilot tones is higher
than the energy on the non-pilot tones.
In FIG. 3, an example set of pilot tones is indicated by the
hatched squares in the time-frequency grid. In this example, the
base station transmits one pilot tone in each OFDM symbol interval.
During: symbol interval (0), tone (1) is used as a pilot tone;
symbol interval (1), tone (4) is used as a pilot tone; symbol
interval (2), tone (0) is used as a pilot tone; symbol interval
(3), tone (2) is used as a pilot tone; symbol interval (4), tone
(4) is used as a pilot tone; symbol interval (5), tone (1) is used
as a pilot tone; etc. . . .
FIG. 4 shows an example of a Latin Squares pilot hopping sequence.
The pilot signal corresponding to a Latin Squares pilot hopping
sequence will be called a "Latin Squares pilot signal", or simply
"Latin Squares pilot". In a Latin Squares pilot hopping sequence,
the number of tones, N, is a prime number, and the pilot signals
are transmitted on a fixed number, N.sub.p, of the N tones in each
OFDM symbol interval. The tone number of the j-th pilot tone in the
t-th symbol interval is given by,
where s and n.sub.j are integers. A Latin Squares pilot signal of
the form of Equation (1) can be viewed as a set of N.sub.p
parallel, cyclically rotating lines in a prescribed time-frequency
grid, i.e., plane. The parameter, s, is the slope of the lines and
the parameters, n.sub.j, are the frequency offsets. In the example
Latin Squares pilot hopping in FIG. 4, N=11, N.sub.p =2, n.sub.1
=1, n.sub.2 =5 and s=2.
The frequency offsets and slope are design parameters of the Latin
Squares pilot signal. For the purpose of channel estimation, the
frequency offsets and slope should be selected so that the pilot
tones are close to uniformly distributed in the time-frequency
plane. A uniform distribution minimizes the worst-case
interpolation error in the channel estimation. Specific values for
the frequency offsets and slopes can be tested by numerical
simulation with a specific channel estimator and channel
conditions.
FIG. 5 depicts an OFDM-SSMA cellular system using Latin Squares
pilots. The figure shows two base stations 501 and 502 in the
cellular system, denoted BS1 and BS2, respectively. Each base
station 501, 502 in the cellular system transmits a Latin Squares
pilot. A mobile user unit 503, denoted MS, receives pilots signals
and other transmissions from one or more base stations in the
cellular system. The Latin Squares pilots transmitted by all the
base stations 501, 502 use the same total number of tones, N,
number of pilot tones per OFDM symbol, N.sub.p, and the frequency
offsets, n.sub.j. However, the slope, s, of each pilot signal is
locally unique in the sense that no two neighboring base stations
use the same slope. Each slope, s, is taken from some set
S.epsilon.{0, 1, . . . , N-1}. The use of locally unique slopes
minimizes collisions between pilot signals from neighboring base
stations. In addition, the slope provides a unique identifier for
each base station. In FIG. 6, the slope of the pilot signal from
BS1 (501) is denoted s.sub.1, and the slope of the pilot signal
from BS2 (502) is denoted s.sub.2.
The base station identification problem is for the mobile user unit
503 to estimate the slope, s.di-elect cons.S, of the strongest
received pilot signal. To perform this identification, the mobile
user unit 503 can be pre-programmed with the common pilot signal
parameters, N, N.sub.p and n.sub.j, as well as the set of possible
slopes, S.
In general, base station identification is conducted prior to
downlink and carrier synchronization. Consequently, a mobile user
unit 503 may receive the pilot signals with unknown frequency and
timing errors, and mobile user units must be able to perform base
station identification in the presence of these errors. Also, after
identifying the pilot hopping sequence of the strongest base
station, the mobile user unit must synchronize its timing and
carrier so that the location of subsequent pilot tones can be
determined.
To define this synchronization problem more precisely, let .DELTA.t
denote the timing error between a base station and mobile user unit
in number of OFDM symbol intervals, and .DELTA.n denote the
frequency error in number of tones. For the time being, it is
assumed that .DELTA.t and .DELTA.n are both integer errors.
Fractional errors will be considered later. Under integer time and
frequency errors, .DELTA.t and .DELTA.n, if a base station
transmits a pilot sequence given by Equation (1), the j-th pilot
tone in the t-th symbol interval of the mobile will appear on tone
number,
where,
and where b(t) is the pilot frequency shift at time t. Equation (2)
shows that if the frequency shift b(t) is known, the locations of
the pilot tones at t are known. Also, if the frequency shift is
determined at any one time, say b(0), the frequency shift at other
times can be determined from b(t)=b(0)+st. Therefore, for
synchronization, it suffices to estimate the frequency shift at any
one time. The value b(0) will be called the initial frequency
shift.
The fact that synchronization requires only the estimation of the
initial frequency shift is a particular and useful feature of the
Latin Square pilot hopping sequences. In general, synchronization
involves estimation of time and frequency errors, and therefore
demands a two parameter search. Synchronization for the Latin
Squares sequences considered here, however, only requires the
estimation of one parameter.
In summary, in an OFDM-SSMA cellular system, each base station
transmits a Latin Squares pilot signal with a locally unique slope.
A mobile user unit performs base station identification by
estimating the slope of the strongest received pilot signal. In
addition, the mobile user unit can synchronize to the pilot signal
by estimating its initial frequency shift.
FIG. 6 shows, in simplified block diagram form, the details of a
mobile user unit 600 containing the proposed maximum energy
detector for base station identification. An incoming signal is
supplied via an antenna 601 to a down conversion unit 602. The
incoming signal includes pilot signals from one or more base
stations. Down conversion unit 602 yields the baseband signal r(t)
from the signal received by the mobile user unit 600. The received
signal r(t) is supplied to fast Fourier transform (FFT) unit 603
that during each OFDM symbol interval performs an FFT on it to
yield Y(t,n). In this example, Y(t,n) denotes the complex value
received on the n-th tone in the t-th symbol interval and is
supplied to maximum energy detector 604 and to receiver 605.
Maximum energy detector 604 uses FFT data Y(t,n) from N.sub.sy
consecutive OFDM symbols to estimate the slope and initial
frequency shift of the pilot signal with the maximum received
strength. As indicated above, the FFT symbols to be used for the
base station identification are denoted Y(t,n), t=0, . . . ,
N.sub.sy -1 and n=0, . . . , N-1, and the estimates of the slope
and initial frequency shift of the strongest received pilot signal
are denoted s and b.sub.0,respectively. The pilot slope s and
initial frequency shift b.sub.0 estimates are supplied to a
receiver 605 and employed to synchronize receiver 605 to the
incoming carrier and to locate subsequent symbols in the pilot
signal.
FIG. 7 shows, in simplified block diagram form details of an
embodiment of a maximum energy detector 604 that may be employed in
the mobile user unit 600 of FIG. 6. It has been seen that for the
Latin Squares pilot tones, each candidate slope, s, and initial
frequency shift, b.sub.0 =b(0), corresponds to a set of predicted
pilot tone locations, (t,n), with
Symbols on these pilot tones should be received with greater power
than the symbols on the non-pilot tones. That is, the energy,
.vertline.Y(t,n).vertline..sup.2, should on average be highest on
the pilot tones of the pilot signal with the strongest received
signal strength. Therefore, a natural way to estimate the slope and
frequency shift of the strongest pilot signal is to find the slope
and frequency shift for which there is a maximum received energy on
the predicted set of pilot tone locations of Equation (4). The
input to the maximum energy detector 604 of FIG. 6 is the FFT data,
Y(t,n), t=0, . . . , N.sub.sy -1 and n=0, . . . ,N-1. The
slope-shift accumulator 701, accumulates the energy along each
possible slope, s, and initial frequency shift, b.sub.0. The
accumulated energy is given by the signal: ##EQU1##
Then, frequency shift accumulator 702 accumulates the energy along
the pilot frequency shifts, namely: ##EQU2##
Maximum detector 703 estimates the slope and frequency shift of the
maximum energy pilot signal as the slope and frequency shifts
corresponding to the maximum accumulated pilot energy, that is:
##EQU3##
where the maximum is taken over s.epsilon.S and b.sub.0 =0, . . . ,
N-1.
Unfortunately, in certain applications, the above computations of
Equations (5), (6) and (7) may be difficult to perform in a
reasonable amount of time with the processing power available at
the mobile user unit 600. To see this, note that to compute J.sub.0
(s,b.sub.0) in Equation (5) at a single point (s,b.sub.0) requires
N.sub.sy additions. Therefore, to compute J.sub.0 (s,b.sub.0) at
all (s,b.sub.0) requires NN.sub.sl N.sub.sy additions, where
N.sub.sl is the number of slopes in the slope set S. Similarly,
computing J(s,b.sub.0) in Equation (6) requires NN.sub.sl N.sub.p
additions. Therefore, the complete energy detector would require
O(NN.sub.sl (N.sub.p +N.sub.sy)) basic operations to perform.
Therefore, for typical values such as N=400, N.sub.sl =200, N.sub.p
=10 and N.sub.sy =20, the full energy detector would require 2.4
million operations. This computation may be difficult for the
mobile user unit 600 to perform in a suitable amount of time.
FIG. 8 shows, in simplified block diagram form details of another
embodiment of a maximum energy detector that may be employed in the
mobile user unit of FIG. 6. Symbolwise shift detector 801
estimates, at each time t, the frequency shift of the pilot signal
with strongest received strength. Specifically, the block computes:
##EQU4##
where E(t) is the maximum energy value and n(t) is the argument of
the maximum. To understand the purpose of the computation in
Equation (8), suppose that the tones of the strongest energy pilot
signal appear at the locations, (t,n), given in Equation (4). Since
the received energy .vertline.Y(t, n).vertline..sup.2, will usually
be maximum at these pilot tone locations, the maximization in
Equation (9) will typically result in:
and E(t) will typically be the pilot signal energy at the time t.
The value n (t) in Equation (9) is precisely the frequency shift
estimate of the pilot signal at time t. Note that n(t) is sometimes
referred to as the symbolwise frequency shift estimate.
Slope-shift solver 802 uses the relation in Equation (9) and the
frequency offset estimates, n(t), to determine the unknown slope,
s, and initial frequency shift, b.sub.0. Since, the pilot signals
are only on average higher in power than the non-pilot tones, the
relation of Equation (9) may not hold at all time points t.
Therefore, the slope-shift solver 802 must be robust against some
of the data points n(t) not satisfying Equation (9). For
robustness, the value E(t) can be used as measure of the
reliability of the data n(t). Larger values of E(t) imply a larger
amount of energy captured at the frequency shift estimate, n(t),
and such values of n(t) can therefore be considered more
reliable.
One possible way of implementing a robust slope-shift solver 802 is
referred to as the difference method. This method uses the fact
that if n(t) and n(t-1) both satisfy Equation (10), then
n(t)-n(t-1)=s. Therefore, the slope, s, can be estimated by:
##EQU5##
where 1 is the indicator function. The estimator as defined by
Equation (10) finds the slope, s, on which the total received pilot
energy, E(t), at the points, t, satisfying n(t)-n(t-1)=s is
maximized. After estimating the slope, the initial frequency shift
can be estimated by: ##EQU6##
The difference method is the process given by Equations (10) and
(11).
A second possible method for the slope-shift solver 802 is referred
to as the iterative test method. FIG. 9 is a flow chart
illustrating the steps for the iterative test solver
Step 901: Start process.
Step 902: Initialize T={0, . . . , N.sub.sy -1}, and E.sub.max
=0.
Step 903: Compute ##EQU7##
where E.sub.0 is the value of the maximum, i.e., strongest value,
and s.sub.0 is the argument of the maximum.
Step 904: If E.sub.0 >E.sub.max, go to step 905.
Step 905: Set
Step 904: If not go to step 906.
Step 906: If T is non-empty return to step 903, otherwise END via
step 907.
The values s and b.sub.0 in Step 905 are the final estimates for
the slope and initial frequency shift of the strongest pilot
signal.
The logic in the iterative test method is as follows. The set T is
a set of times and is initialized in Step 902 to all the N.sub.sy
time points. Step 903 then finds the time, t.sub.0.epsilon.T, and
slope, s.sub.0.epsilon.S, such that the set of times t on the line
n(t)=n(t.sub.0)+s.sub.0 (t-t.sub.0), has the largest total pilot
signal energy. The points on this line are then removed from T. In
Step 904, if the total energy on the candidate line is larger than
any previous candidate line, the slope and frequency shift
estimates are updated to the slope and frequency shifts of the
candidate line in step 905. Steps 903 through 906 are repeated
until all points have been used in a candidate line.
Both the difference method and iterative test method demand
significantly less computational resources than the full maximum
energy detector. In both methods, the bulk of the computation is in
the initial symbolwise shift detection in Equation (8). It can be
verified that to conduct this maximization at all the N.sub.sy time
points N.sub.sy NN.sub.p operations. Therefore, for the values
N=400, N.sub.p =10 and N.sub.sy =20, the simplified maximum energy
detector would require 80000 operations, which is considerably less
than the 2.4 million needed by the full energy detector.
The above base station identification methods can be further
simplified by first quantizing the FFT data Y(t,n). For example, at
each time t, we can compute a quantized value of Y(t,n) given by:
##EQU8##
where q>1 is an adjustable quantization threshold, and .mu.(t)
is the mean received energy at time t: ##EQU9##
The quantized value Y.sub.q (t,n) can then be used in place of
.vertline.Y(t,n).vertline..sup.2 in the above base station
identification processes. If the parameter q is set sufficiently
high, Y.sub.q (t,n) will be zero at most values of n, and therefore
the computations such as Equation (8) will be simplified.
In the above discussion, it has been assumed that the time error
between the base station and mobile is some integer number of OFDM
symbol intervals, and the frequency error is some integer number of
tones. However, in general both the time and frequency errors will
have fractional components as well. Fractional errors result in the
pilot tones being split between two time symbols and spread out in
frequency. This splitting reduces the pilot power in the main
time-frequency point, making the pilot more difficult to identify.
Meanwhile, without proper downlink synchronization, data signals
from the base station are not received orthogonally with the pilot
signal, thus causing extra interference in addition to that
generated by neighboring base-stations. Overall, fractional time
and frequency errors can thereby significantly degrade the base
station identification. In particular, the strongest energy
detection process may not perform well.
To avoid this fractional problem, the above identification
processes be run at several fractional offsets. Specifically, for a
given received signal r(t), the mobile user unit can slide the FFT
window N.sub..function.r,t times along the time axis, each time
obtaining a different set of frequency sample vectors. The step
size of sliding the FFT window should be 1/N.sub..function.r,t of
the symbol interval. Similarly, the mobile user unit can slide the
FFT window N.sub..function.r,.function. times along the frequency
axis with a spacing of 1/N.sub..function.r,.function. of a tone.
The identification process can be run on the frequency samples
obtained from each of the fractional time and frequency offsets.
This process yields N.sub..function.r,t N
.sub..function.r,.function. candidate slope and frequency
shifts.
To determine which of the N.sub..function.r,t N
.sub..function.r,.function. candidate slope and shifts to use, the
mobile user can select the slope and shift corresponding to the
strongest pilot energy. For a given candidate (s,b.sub.0) the pilot
energy is given by J(s,b.sub.0) in Equation (6). If the difference
method is used, an approximation for the pilot energy is given by
the value of the strongest attained in equation (11). The value
E.sub.max may be employed in the iterative test method.
The above-described embodiments are, of course, merely illustrative
of the principles of the invention. Indeed, numerous other methods
or apparatus may be devised by those skilled in the art without
departing from the spirit and scope of the invention.
* * * * *